Sacrificial anode assembly for vessel interiors

ABSTRACT

Sacrificial anode assemblies for the protection of vessel interiors are provided with means for adjusting anode surface exposure to maintain a uniformly protective potential. A packing gland assembly positioned within a port in a vessel wall allows adjustment of anode exposure from the outside of the vessel. The sacrificial anode is in the form of a rod which may be moved in or out against the frictional engagement of a seal ring compressed within the packing gland.

May 2, 1972 'K. w. SCHULLER 3,550,254

SACRIF'IGIAL ANODE ASSEMBLY FOR VESSEL INTERIORS Filed July 22, 1970 55 if g I 15 10 12 16s 5g 14 Q \1 M 17 5 16 g w O & p v 22 13 Z8 54 i w} L z @Y' I ZI INVEN'fOR. K0 r/ Wi/be/m Schufler x4 TTORNEY United States Patent 01 flee 3,660,264 SACRIFICIAL ANODE ASSEMBLY FOR VESSEL INTERIORS Karl Wilhelm Schuller, Lake Jackson, Tex., assignor to The Dow Chemical Company, Midland, Mich. Filed July 22, 1970, Ser. No. 57,267 Int. Cl. C23f 13/00 US. Cl. 204-197 5 Claims ABSTRACT OF THE DISCLOSURE It is known that the corrosion of interior walls of vessels employed to confine liquids can be reduced or eliminated by impressing a voltage on the walls so as to render them cathodic relative to a second electrode in contact with the aqueous system. Such voltages may be applied directly from a primary source of electrical energy. One technique, however, which is frequently more convenient and efficient involves introducing a second metal, which is anodic to the vessel walls, into the corroding liquid environment. By anodic it is meant that the metal will dissolve, or corrode, in the liquid preferably to the metal walls to be protected when galvanically connected thereto. The sacrificial anode has the effect of generating the protective electromotive force in situ.

One difiiculty with the use of the sacrificial anode, however, is the variation in surface area presented to the corroding liquid medium. As this surface area decreases, due to corrosion, the protective voltage declines. This problem is particularly aggravated in heat exchangers, such as reboilers, water heaters and water evaporators, wherein highly corrosive saline water systems are encountered.

It is an object of the instant invention to provide a sacrificial anode system whereby the protective potential generated may be controlled to uniformly protective levels over extended periods of time.

A further object is to providea sacrificial anode assembly for use in vessels, which may be adjusted from outside the vessel.

Another object is to provide an improved means for protecting heat exchanger walls from the corrosive effects of saline aqueous systems.

Still another object is to provide a sacrificial anode assembly which is conveniently installed and readily renewed.

The accompanying drawing depicts a sacrificial anode assembly fabricated in accordance with one embodiment of the instant invention. The assembly is depicted as being installed within the header wall of a single pass heat exchanger.

SUMMARY OF THE INVENTION In one embodiment, the invention comprises a sacrificial anode assembly projecting through a port in a wall of a metal vessel utilized to confine a body or flowing stream of an ionically conducting, corrosive liquid. The interiorly disposed portion of the anode assembly positions the anode in contact with the liquid 3,660,264 Patented May 2, 1972 contents of the vessel when it is in use. The anode assembly comprises a rod of metal anodic to the metal wall and other components of the vessel to be protected. This anodic rod extends through a packing gland assembly of concentric tubular members. The outside tubular member has a shoulder flange which seats against the outside facing of the vessel wall and an interior retaining flange which provides a seat for a flexible seal ring. The inside member of the packing gland butts, at one end, against the seal ring seated within the outside tubular member and, at the opposite end, extends beyond the outside gland member to provide an abutting-surface against which pressure can be applied to the flexible seal ring. Under pressure, the latter tightly circumscribes, but slidably engages, the anode rod to provide a liquid tight seal. Through this seal, the rod can be adjusted as necessary to maintain uniform anode exposure. The same force that acts on the seal ring also seats the packing gland assembly in the vessel wall. The force necessary to compress the seal ring and seat the entire assembly may be applied to the packing gland by any convenient pressure means such as a nut turned down on a threaded shoulder extending from the vessel walls.

In the drawing, the anode assembly 10 is seated in the header wall 11 of a single path heat exchanger 27 for steam-heating sea water on the tube side of the exchanger 27. The anode assembly 10 comprises a metal anode rod 13 made of an aluminum or magnesium alloy, which is anodic with respect to the header wall 11, tube sheet 28 and tubes 25 of the heat exchanger 27. The anode assembly 10 projects through a port in the header wall 11. The exposed portion 26 of the anode is spaced apart from the header wall 11 sufficiently to avoid charge concentration effects that would occur if such anodes were exposed to the corrosive medium immediately adjacent the materials to be cathodically protected. The optimum distance variesaccording to the design of the vessel but will generally be at least about 10 cm. and preferably positioned, insofar as possible, approximately equidistant from all metal surfaces to be protected.

Maintaining the anode rod 13 in the desired position is a packing gland assembly 29 comprising two concentric, tubular gland members, 15 and 23 respectively. The outside gland member 23 has a shoulder flange 30 at one end and a seal retaining flange 31 at the other. It is preferred to form the outside gland member 23 from electrically insulating materials so as to avoid introducing electrical communication between the anode and the walls to be protected. Suitable materials of construction include the thermoset and thermal-plastic molding resins, preferably reinforced With fiber glass or other electrically insulating fillers. The interior gland member 15 rests at one end of the seal ring 22 and abuts at the other end, through a shoulder flange 32, against pressure applicating means. In this embodiment, pressure is applied to the seal ring 22 through a nut 14 which is turned down on a threaded shoulder 18 extending from the header wall 11 and circumscribing the entrance port for the entire assembly 10. A packing 16 between the nut 14 and shoulder flange 32 provides a second frictional seal and sliding engagement with the anode rod 13. The entire packing gland rests on, and is compressed against, a gasket 17 made of a flexible material, such as rubber or a flexible plastic. Extending downwardly from and firmly fastened to the outside of tubular gland member 23 of the packing gland assembly 29 are optional anode retaining jaws 19 held in a fixed relationship to the gland member 23 by means of a pinching ring 20.

It is the purpose of the retaining jaws 19 to limit the penetration of the anode rod 13 to a maximum depth. This function may be eliminated if desired and thus the jaws are an optional feature of the instant invention. Outward movement of the anode rod 13 is prevented by threads 12 which engage mating threads impressed in the packing 16 and the seal ring 22.

The anode rods have any convenient cross-sectional shape. Preferably, they are round for convenience in maintaining a tight seal. As illustrated by rod 13' female threads may be provided in the exteriorly exposed end of the rod for convenient connection with a replacement rod. Materials from which sealing rings 22 and packings 16 may be prepared, include for example, rubber, polyperfiuoroethylene, acrylonitrile-butadiene copolymer plastics and soft metals. The anode rods 13 will usually be constructed of magnesium or aluminum alloys, many of which are specially doped with other metals to minimize passivation and to enhance the potential generated. The particular type of metal utilized, however, is not critical to the invention so long as it is anodic, i.e. dissolves, relative to the metal components to be protected.

In the practice of the invention, the circuit is completed by means of ground connections 33 and 34. The current generated by the dissolving anode may be monitored as with a microammeter 35. When the potential begins to decrease, the anode is inserted further into the liquid contained within the vessel. Optionally, ground connections 33 and 34 can be replaced with a direct electrical connection from the anode rod to the metal to be protected. If close control of anode movement is desired, the anode may be threaded on its outside surface, as illustrated, and at least one of the packing rings provided with threads to provide means for screwing it into, or out of, the vessel. Alternatively, the threads may be omitted and such movement may be simply accomplished by directly forcing the anode rod through frictional seals.

In large installations, it will be convenient to maintain remote surveillance of the potential generated. As the potential decreases to unsafe levels, the necessary adjustments are made to restore the desired protective potential.

What is claimed is:

1. A sacrificial anode assembly in combination with, and projecting through a port in, a wall of a metal vessel,

the interiorly exposed portion of the anode being positioned Within the vessel so as to contact liquid contents of the vessel at a position spaced apart from the vessel walls, said anode assembly comprising a sacrificial anode rod extending through a packing gland assembly of concentric tubular members, the outside tubular member thereof having a shoulder flange to seat against the outside facing of the vessel wall and an interior retaining flange on which a flexible seal ring tightly circumscribing the anode rod is seated and an inside tubular gland member abutting against the seal ring at one end and at the other end against pressure applying means fastened to the outside facing of the vessel wall.

2. The combination of claim 1 wherein the pressure applying means is a nut which screws onto a threaded shoulder extending from the outside facing of the vessel wall.

3. The combination of claim 2 wherein the pressure applying means and the abutting surface of the inner tubular gland member confine a packing in sliding engagement with the anode rod.

4. The combination of claim 1 wherein the packing gland assembly extends into the vessel sufficiently that the exposed anode rod is in a zone substantially equidis tant from the metal surfaces to be protected.

5. The combination of claim 1, and including in addition, retaining jaws to limit interior penetration of the anode rod.

References Cited UNITED STATES PATENTS 2,808,373 10/1957 Andrus 204-197 2,816,069 12/ 1957 Andrus 204196 2,949,417 8/1960 Preiser et a1. 204-496 3,133,873 5/1964 Miller et a1. 204-196 TA-HSUNG TUNG, Primary Examiner U.S. Cl. X.R. 

